fix(mcp): extend env drift guard to examples and scripts directories (#413)

[sparkysolutions10-pixel]

Summary

Closes #413

Extends the repository environment drift guard to scan examples/ and scripts/ directories in addition to src/, ensuring all environment variables (including dangerous mainnet secret keys) are documented in .env.example with clear operational warnings.

Changes

• Expanded Drift Guard: Updated src/__tests__/envDrift.test.ts to scan source files across src/, examples/, and scripts/.
• Documented Variables & Warnings in .env.example:
  • Added explicit documentation and warnings for MCP_AGENT_SECRET_KEY_MAINNET stating plainly that it signs transactions on the public network and can spend real funds.
  • Documented ORACLE_RELAY_RPC_URL and ORACLE_RELAY_NETWORK_PASSPHRASE used in examples/oracle-relay/relay.ts.
• Negative Test Case: Added unit test in src/__tests__/envDrift.test.ts verifying that any un-documented process.env read in examples/ fails the drift guard as expected.

Tests

• Run npx vitest run src/__tests__/envDrift.test.ts:
  • asserts each process.env read in source appears in .env.example or the allow-list: PASS
  • fails when a new un-documented process.env read is encountered in src/: PASS
  • fails when an un-documented process.env read is encountered in examples/ (#413): PASS
  • requires every allow-list entry to have an explicit rationale: PASS